1. Field of the Invention
This invention relates generally to an optical transmission system, and, more particularly, to a method and apparatus for transforming optical wave modes in an optical wave-guide transmission system.
2. Description of the Related Art
Photonics, the use of light to store, transmit, and/or process information, is rapidly penetrating the market for commodity and high technology products. For example, optics is the transmission medium of choice for many metropolitan and local-area networks. Lasers are often used to provide light to photonic devices, which may be optically coupled to the laser by a waveguide. The waveguide transfers the light from the laser to the photonic device and, in some cases, may act as an impedance matching device and/or a filter. For example, many efforts are directed to closely integrating laser sources, detectors, and other microelectronic devices with optical devices formed using wave-guide technology.
FIGS. 1A-C conceptually illustrate a conventional process flow for forming a waveguide 100, which includes a lower buffer layer 105, a waveguiding element 110, and an upper buffer layer 115, as shown in FIG. 1C. In various embodiments, the lower buffer layer 105, the waveguiding element 110, and the upper buffer layer 115 may be formed using materials having optical qualities appropriate to the particular application. Referring now to FIG. 1A, a layer of waveguiding material 120 having a vertical dimension Tw is deposited over the lower buffer layer 110. A photoresist mask 125 having a horizontal dimension Dp and a vertical dimension Tp is then formed over the layer of waveguiding material 120. The horizontal dimension Dp approximately corresponds to the desired horizontal dimension Dw (shown in FIG. 1B) of the waveguiding element 110 formed by the etching process. The vertical dimension Tp of the photoresist mask 125 is determined, at least in part, by the desired anisotropy of the etching process and/or the vertical dimension Tw.
An etchant 130 is provided to remove exposed portions of the layer of waveguiding material 120. For example, reactive ion etching techniques may be used to remove exposed portions of the layer of waveguiding material 120. Once the etching is complete, the photoresist mask 125 is removed, leaving the waveguiding element 110, as shown in FIG. 1B. The upper buffer layer 115 may then be formed over the waveguiding element 110 and portions of the lower buffer layer 105. For example, the upper buffer layer 115 may be conformally deposited over the waveguiding element 110 and the lower buffer layer 105.
The waveguide 100 described above may suffer from a number of drawbacks, particularly when used to transfer light modes from a laser to a photonic device. The horizontal dimension Dw of the etched waveguiding element 110 is determined, at least in part, by the horizontal dimension Dp, which may in turn be determined, at least in part, by the vertical dimension Tp, which, as described above, depends upon Tw. Thus, constraints imposed by the desired anisotropy of the etching process and/or the vertical dimension Tw of the layer of waveguiding material 120 may limit the horizontal dimension Dw of the etched waveguiding element 110. For example, conventional etched waveguiding elements 110 have horizontal dimensions Dw of about 4-6 microns.
Moreover, the photoresist mask 125 may erode during the etching process. Since the duration of the etching process is approximately proportional to the vertical dimension Tw of the layer of waveguiding material 120, the amount of the photoresist mask 125 that may erode during the etching process may be approximately proportional to the vertical dimension Tw. Thus, increasing the vertical dimension Tw may increase erosion of the photoresist mask 125. Erosion of the photoresist mask 125 may lead to a corresponding reduction in the accuracy with which the etching process may transfer the pattern of the photoresist mask 125 to the layer of waveguiding material 120.
Furthermore, modal confinement factors of the laser, the wave-guide, and the transmission systems fiber may be different. For example, the laser modes may be characterized by strong linear polarization, normally in a plane horizontal to the device, and the laser modes at an output facet of the laser may be highly elliptical and have an anisotropic divergence. The highly elliptical modes at the output facet of the laser are difficult to transform into the typical polarization modes of a standard single mode fiber. Consequently, it may be difficult to couple, with low loss, the laser to the standard single mode fiber. In conventional packaged assemblies, anamorphic lenses are used to optimize launch efficiencies. Alternatively, modal transformation of the laser spot is attempted using optical devices that are monolithically integrated into the laser structure. However, the complexity and consequent cost incurred by including anamorphic lenses, monolithically-integrated optical devices, and the like is highly undesirable from a manufacturing perspective.